A rubber such as a natural rubber, a styrene-butadiene rubber, and a chloroprene rubber has generally been used for a friction transmission belt for power transmission used for an automobile engine and the like. However, with the social needs for energy conservation and space saving in the background, there is a tendency of densely packing component parts in an engine room of an automobile, and an ambient temperature in the engine room is being increased due to the dense packing as compared to the conventional examples.
In such high temperature environment, a problem of an early crack generation that occurs due to hardening of the rubber used for the friction transmission belt has been pointed out. Also, since a revolution fluctuation of the engine has been increased along with the energy conservation, a tension fluctuation of a power transmission belt is increased due to influence of the rotation fluctuation, thereby causing a problem of early wear and noise generation. Further, since a rubber containing halogen such as the chloroprene rubber leads to generation of dioxin, a belt made from a rubber free from halogen which is an environmental load substance has been in demand recently.
In relation to such demand, ethylene•α-olefin rubbers such as an ethylene-propylene rubber (EPM) and an ethylene-propylene-diene copolymer rubber (EPDM) have recently been noted as promising rubbers since they have excellent heat resistance, are relatively cheap polymers, and satisfy the demand of dehalogenation. Specifically, a friction transmission belt for power transmission, which uses an ethylene•α-elastomer reinforced by a metal salt of an α-β-unsaturated organic acid has been proposed (see, for example, Reference 1).
Also, there has been raised a problem of noise of belt which is caused by an increase in belt approach angle due to low fuel consumption and space-saving of engine. Particularly, a belt friction coefficient is largely changed due to use conditions such as a dry state, a wet state, and a pulley coating type to cause the noise.
As a countermeasure against the noise, a friction transmission belt obtainable by adding various additives of low friction properties to a rubber composition forming a friction transmission surface and capable of suppressing the noise generation has been proposed. For example, Reference 2 proposes a friction transmission belt obtainable by adding an oil to a belt made from a polyurethane elastomer.
[Reference 1] JP-A-9-500930
[Reference 2] JP-A-11-264447
However, since the ethylene•α-olefin elastomer is poor in wettability with water and easily repels water as compared to the chloroprene rubber, a water intrusion state between a belt and a pulley in the wet state is not uniform. A friction coefficient is not reduced and the belt is brought into tight contact with the pulley at a portion which is not intruded with water, while a friction coefficient is partially reduced to cause slipping between the belt and the pulley at a portion intruded with water, thereby raising a problem of being subject to stick-slip noise. However, in the case of rearward running, since the ethylene•α-olefin elastomer is inferior in wettability to the chloroprene rubber, a problem that transmission performance is deteriorated or that a noise is generated by slipping when rain or mad water intrudes (in the wet state) into the pulley section has been raised.
Also, in the case of adding the low friction coefficient substances to a rubber composition, the belt friction coefficient is changed depending on the use conditions such as the dry state, the wet state, and the pulley coating type, thereby raising a problem of failing to achieve stable noise prevention.